DC motor has been widely used in the field of motion control because of its excellent torque characteristics. However, the ordinary DC motor needs mechanical commutation and brush, which has poor reliability and needs frequent maintenance. Electromagnetic interference and noise will be generated during commutation, which will affect the further application of DC motor in control system. In order to overcome the shortcomings caused by mechanical commutation, the brushless motor with electronic commutation instead of mechanical commutation came into being. 1955, American D. Harrison and others applied for the patent of replacing mechanical brush with transistor commutation circuit for the first time, which marked the birth of modern brushless motor. 1978, the classic brushless DC motor for MAC and its driver came out, and the electronically commutated brushless DC motor really entered the practical stage. After that, the brushless DC motor has been deeply studied in the world, and it has developed into square wave brushless DC motor and sine wave brushless DC motor. In recent 20 years, with the development of new permanent magnet materials, microelectronics technology, automatic control technology and power electronics technology, especially the development of high-power switching devices, brushless motors have made great progress. Brushless DC motor is not only an electronically commutated DC motor, but also an electronically commutated motor with external characteristics of brush DC motor [1].
Brushless DC motor not only maintains the good dynamic and static speed regulation characteristics of traditional DC motor, but also has simple structure, convenient operation and easy control. Its application has rapidly developed from the initial military industry to aerospace, medical care, information, home appliances, industrial automation and other fields.
Structurally, unlike brushless DC motor, the stator winding of brushless DC motor is used as armature, and the excitation winding is replaced by permanent magnet material. DC brushless motors can be divided into square-wave DC motors (BLDCM) and sine-wave DC motors (PMSM) according to different current waveforms flowing into armature windings. BLDCM replaces the mechanical commutation of DC motor with electronic commutation, and uses permanent magnet as rotor, which saves brushes. On the other hand, PMSM replaces the excitation winding in the rotor of synchronous motor with permanent magnet material, which saves the excitation winding, slip ring and brush. Under the same conditions, the drive circuit is easier to obtain square waves and the control is simple, so the application of brushless DC motor is much wider than that of PMSM [2].
Brushless DC motor generally consists of three parts: electronic commutation circuit, rotor position detection circuit and motor body. Electronic commutation circuit is generally composed of control part and driving part, and the detection of rotor position is generally completed by position sensor. When working, the controller sequentially triggers each power tube in the drive circuit to carry out orderly commutation according to the position of the motor rotor measured by the position sensor, and drives the DC motor [3]. This paper analyzes the development of brushless motor from three parts.
2 the development of each component
2. 1 motor body
The electromagnetic structure of brushless DC motor is basically the same as that of brushless DC motor, but its armature winding is placed on the stator, and the weight adopted by the rotor simplifies the structure, improves the performance and improves the controllability. The development of brushless motor is inseparable from the development of permanent magnet materials. The development process of magnetic materials has basically gone through the following stages: Al-Ni-Co, ferrite magnetic materials and NdFeB. NdFeB is a kind of high magnetic energy product, and its appearance has caused a revolution in magnetic materials. The application of the third generation NdFeB permanent magnet material further reduces the amount of copper used in the motor and promotes the development of brushless motor in the direction of high efficiency, miniaturization and energy saving [4].
At present, in order to improve the power density of the motor, a transverse magnetic field permanent magnet motor has appeared. The stator cogging and armature coil are perpendicular to each other in space, and the main magnetic flux in the motor flows along the motor axis. This structure improves the air gap magnetic density and can provide much larger output torque than the traditional motor [5]. This type of motor is in the research and development stage.
2.2 electronic commutation circuit
Control circuit: Brushless DC motor controls the speed, steering and torque of the motor by controlling the power switch devices in the driving circuit, and protects the motor, including overcurrent, overvoltage and overheating protection. The control circuit initially adopts analog circuit, and the control is relatively simple. If the circuit is digitized, many hardware tasks can be directly completed by software, which can reduce the hardware circuit, improve its reliability and improve the anti-interference ability of the control circuit, so the control circuit has developed from analog circuit to digital circuit.
At present, there are three kinds of control circuits: ASIC, microprocessor and digital signal processor. It is a simple and practical method to form the control circuit with professional integrated circuits under the condition of low requirements for motor control. Digital signal processor is the development direction of control circuit because of its fast operation speed, few peripheral circuits and simple system composition. The composition of DC brushless motor is greatly simplified and its performance is greatly improved, which is conducive to the miniaturization and intelligence of motor.
Drive circuit: the drive circuit outputs electric power and drives the armature winding of the motor, which is controlled by the control circuit. The driving circuit consists of high-power switching devices. It is precisely because of the appearance of thyristor that DC motor has achieved a leap from brush to brushless. However, thyristor is a semi-controlled switching device, which only has the ability of controlling conduction, but has no ability of self-turning-off, and the switching frequency is low, which can not meet the further improvement of brushless DC motor performance. With the rapid development of power electronics technology, fully controlled power switching devices have emerged, including turn-off transistor (GTO), power field effect transistor (MOSFET), metal gate bipolar transistor IGBT module, integrated gate commutated thyristor (IGCT) and newly developed electron injection enhanced gate transistor (IEGT) [7]. With the continuous improvement of the performance of these power devices, the corresponding brushless motor drive circuit has also developed rapidly. At present, fully-controlled switching devices are gradually replacing ordinary thyristors with complex circuits, huge volume and low functional indexes. The driving circuit is changed from linear amplification state to pulse width modulation switching state, and the corresponding circuit composition is also changed from power tube discrete circuit to modular integrated circuit, which creates conditions for the intelligent, high-frequency and miniaturization of the driving circuit.
2.3 Rotor position detection circuit
Permanent magnet brushless motor is a closed-loop mechatronics system, which uses the rotor pole position signal as the commutation signal of electronic switch circuit. Therefore, it is the key to detect the rotor position accurately and switch the power devices in time according to the rotor position.
It is the most direct and effective method to use the position sensor as the rotor position detection device. The position sensor is generally installed on the shaft of the rotor to realize real-time detection of the rotor position. The earliest position sensor is magnetoelectric, heavy and complicated, and has been eliminated. At present, magnetic Hall position sensors and photoelectric position sensors are widely used in brushless DC motors. The existence of position sensor increases the weight and structural size of brushless DC motor, which is not conducive to the miniaturization of motor; When rotating, the sensor is inevitably worn and difficult to maintain; At the same time, the installation accuracy and sensitivity of the sensor directly affect the running performance of the motor; On the other hand, because there are too many transmission lines, it is easy to introduce interference signals; Because the signal is collected by hardware, the feasibility of the system is further reduced. In order to adapt to the further development of brushless motor, sensorless came into being. Generally, the position of rotor magnetic pole is obtained indirectly by using the back electromotive force induced by armature winding. Compared with the direct detection method, it saves the position sensor, simplifies the structure of the motor, achieves good results and is widely used. However, for the sensorless brushless motor that detects the position of * back electromotive force, how to start it smoothly is a problem that needs to be solved because it does not produce back electromotive force when it is at rest.
In recent years, a new sensorless brushless motor is proposed, which does not use back electromotive force to detect the rotor position, but uses the non-magnetic material attached to the rotor surface and the eddy current effect on the non-magnetic material when the stator winding high-frequency switch works, so that the open-circuit phase voltage changes with the rotor position, so that the rotor position can be judged by detecting the open-circuit phase voltage. This sensorless brushless motor overcomes the problems of start-up and low-speed operation of general sensorless brushless motors, but this method requires a special motor.
3 Problems to be studied
3. 1 torque fluctuation
At present, the main problem of brushless DC motor is torque ripple. Due to the existence of torque ripple, the application of brushless DC motor in AC servo system is limited, especially in the case of direct drive application, torque ripple makes the speed control characteristics of the motor worse. Brushless DC motors, especially those used in audio-visual equipment, film machinery and computers, require smooth operation and no noise. Therefore, restraining or eliminating torque ripple is the key to improve the performance of servo system.
The main causes of torque ripple are: cogging effect and torque ripple caused by flux distortion; Torque ripple caused by harmonics; Torque ripple caused by commutation current due to the influence of armature equivalent inductance. At present, universities and scientific research institutions have conducted in-depth research on torque ripple, and put forward various methods to suppress or weaken torque ripple according to different reasons, thus improving the performance of brushless motors to varying degrees. However, these studies have put forward some methods to weaken or compensate the original structure and scheme, and have not eliminated the torque ripple in principle or fundamentally. Therefore, torque ripple needs further study.
3.2 Rotor position detection without position sensor
The rotor position detection methods without position sensor mainly include back electromotive force method, free-wheeling diode method, inductance method and state observation method. Among them, the back electromotive force method is the most common and widely used method. However, on the basis of ignoring armature reaction, this method has errors in principle. For high-power brushless motor, armature reaction has more obvious influence on air gap magnetic density, and the error is bigger. On the other hand, when the motor starts at a low speed, the back electromotive force is zero or very small, so it is difficult to detect the rotor position through the back electromotive force, and the brushless motor without position sensor has a starting problem [9]. Therefore, how to compensate the rotor position signal error caused by back electromotive force method in high-power brushless motor and how to overcome the starting problem of motor in back electromotive force method are urgent problems to be solved. For the start-up problem, it is generally started by other methods, and then switched to the sensorless operation mode.
4 development direction of brushless DC motor
With the development of electronic technology and control technology, position detection can be realized by matching appropriate algorithms with chips. The appearance of high-speed microprocessor, DSP device and special control chip has greatly improved the running speed and processing capacity. Sensorless control can be realized on brushless motor by using the inherent computing power of DSP [10]. Using DSP to realize sensorless control has become a research hotspot, and low-cost DSP sensorless brushless motor has become the development direction of brushless DC motor.